Large volumes of natural gas (i.e., primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Where the gas reserves are located in reasonable proximity to a market and the terrain between the two locations permits, the gas is typically produced and then transported to market through submerged and/or land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas (“LNG”). LNG is typically, but not always, transported at substantially atmospheric pressure and at temperatures of about −162° C. (−260° F.), thereby significantly increasing the amount of gas which can be stored in a particular storage tank on a transport vessel. For example, LNG takes about 1/600 of the volume of natural gas in the gas phase.
Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like. LNG has been an increasingly popular transportation method to supply major energy-consuming nations with natural gas.
The liquefaction process may have a number of stages during which the natural gas is cooled and liquefied. During the cooling process, the pressure is lowered, with the shipping pressure of the liquefied product being near atmospheric (for example, about 3.6 psig or less). The decrease in pressure assists in cooling the natural gas during the liquefaction process by decreasing the enthalpy of the natural gas. Refrigeration equipment is also used for removing heat energy.
One stage of this process requires that the high-pressure liquid phase of the natural gas stream be reduced in pressure sufficiently to assist in the production of extremely cold LNG (or subcooled LNG) by extracting energy (or enthalpy) from a liquid natural gas stream. This may be accomplished through hydraulic turbine pressure drop.
Hydraulic turbine pressure drop can often be used in LNG processes to remove energy from liquid refrigerant streams and liquid natural gas streams to obtain lower temperatures. The energy removed from these liquid streams may also be used to generate electrical power. For example, turbines can be coupled with a generator to provide the braking load necessary to remove the energy. The generator may be coupled to the facility power grid, wherein the additional power improves the thermodynamic efficiency of the process. In LNG processes, the efficiency improvement may be about 1 to 2%, resulting in saving many Megawatt-hours per year and improving economic justification of the liquefaction process.
Other parties have proposed the concept of applying turbines in series to satisfy the need for high pressure let down at a magnitude greater than typically performed in existing facilities. Examples of series expansion are considered in patents related to air separation, as well as in cascade LNG liquefaction processes, among others.
U.S. Pat. No. 3,724,226 to Pachaly discloses an LNG expander cycle process employing integrated cryogenic purification. In the process, a work-expanded refrigerant portion undergoes a compression cycle and is work expanded through a series of expansion turbines. The expansion turbines furnish at least part of the power necessary to drive the compressor system in the refrigerant gas cycle, by sharing a common shaft or other mechanical coupling with the compressors. The expanders used are turbo-expanders, which can liquefy a portion of a high-pressure gas stream as it is depressurized through the turbo expanders. The expanded stream can then be flowed through cooling units to remove more energy, prior to flowing through more turbo-expanders.
U.S. Pat. No. 4,019,343 to Roberts discloses a refrigeration system using enthalpy converting liquid turbines. The refrigeration system uses a series of liquid turbines, each of which have an associated compressor. A stream of liquid ammonia is allowed to expand in a liquid turbine, during which a portion of the liquid flashes and is sent to the associated compressor. The cooled, expanded liquid flows to the next turbine in the series, where the process is repeated.
Related information may be found in U.S. Pat. Nos. 2,922,285; 3,677,019; 4,638,638; 4,758,257; 5,651,269; 6,105,389; 6,647,744; 6,898,949; and 7,047,764. Further information may also be found in U.S. Patent Application Publication Nos. 2003/0005698 and 2005/0183452. Additional information may be found in International Patent Application Publication No. WO 2007/021351 and European Patent Application Publication No. 0 672 877 A1.
Due to the increase in demand seen in recent years, increased emphasis has been placed on cost and schedule efficiency of new gas liquefaction projects in order to reduce the cost of the delivered gas. Large natural gas liquefaction projects expose the developers to substantial commercial risk due to the large initial capital costs of these projects (which may for example be $5 billion or higher). Improvements in cost, design, and schedule efficiency can help mitigate the substantial commercial risk associated with large LNG development projects.